The present invention relates to equipment capable of pumping gases or liquids, such as rotary axial fans, propellers, axial compressors, turbines and similar structures and in particular to a fan with reduced noise.
Cooling fans are ubiquitously used to provide forced air cooling of electronic equipment and other sources of heat such as heaters, air conditioners, heat exchangers and automobile engines. When cooling fans are used in a relatively quiet ambient noise environment, such as an office, fan noise is a significant problem. Personal computer cooling fans, for example, may create a majority of the ambient noise in a quiet office environment. This noise may annoy those working in that environment. Thus, the reduction of fan noise may significantly reduce the level of annoying noise experienced by those in a relatively quiet environment. Even in higher ambient noise environments, such as the vicinity of an automobile engine in operation, fan noise may contribute an appreciable amount to the noise level, making the reduction of fan noise a priority.
Fan noise originates from three related types of phenomena: aerodynamic, electromagnetic and mechanical. Aerodynamic phenomena causing fan noise include the blade end vortex, blade wake turbulence, hub turbulence, blade rate tone (siren effect), strut turbulence and hub/blade transition turbulence. Electromagnetic phenomena include magnetic field changes due to commutation, irregularity of the rotor gap and rotor and stator slots. Mechanical phenomena contributing to fan noise include bearing vibration and imbalance, both static and dynamic.
Of the various sources of fan noise previously mentioned, the dominant source is typically the fan blade end vortex, which also reduces the efficiency of the fan. Reduction of the blade end vortex is therefore obviously desirable because it reduces fan noise while increasing fan efficiency.
Therefore, a need has arisen for a fan that addresses the disadvantages and deficiencies of the prior art. In particular, a need has arisen for a fan which produces less noise.
Accordingly, in one aspect of the present invention, a fluid propulsion assembly includes a hub and a plurality of blades. Each blade has an inner edge attached to the hub. A plurality of end-pieces are each attached to an outer edge of a corresponding one of the blades. Each end-piece extends in the axial direction beyond the surface of the outer edge of the corresponding blade. In one embodiment, each end-piece extends in the axial direction by an amount sufficient to substantially prevent the formation a blade end vortex by its corresponding blade. In another embodiment, each end-piece extends upstream and downstream of the corresponding blade by an amount equal to at least three times the profile thickness of the corresponding blade.
In another aspect of the present invention, a fan includes a housing, a hub rotatably coupled to the housing, a plurality of fan blades, each with an inner edge attached to the hub, and a plurality of end-pieces. Each end-piece is attached to an outer edge of a corresponding one of the fan blades. Each end-piece extends in the axial direction beyond the surface of the outer edge of the corresponding fan blade.
An advantage of the present invention is that less turbulence is created by the fan blades, resulting in reduced fan noise and greater fan efficiency.